Active Power: In a DC circuit, the electrical power delivered from the power source to the electrical device (load) is the product of voltage and current, representing the actual power absorbed by the device. In an AC circuit, due to the simultaneous presence of resistance and reactance (inductive and capacitive reactance), the electrical power delivered from the power source to the device does not entirely perform work. Since a portion of the electrical power (energy stored in inductors and capacitors) can still be fed back to the power source, the actual power absorbed by the device is referred to as active power. It is denoted by the letter P. The international unit is the watt (W). Typically, active power is expressed in kilowatts (kW).
Reactive Power: The electrical energy stored in inductors and capacitors can still be fed back to the power source. This portion of power exchanges between the power source and reactance without being consumed, and is termed reactive power. It is denoted by the letter Q. The international unit is the var (var). Typically, reactive power is expressed in kilovars (kvar).
Inductive Reactive Power: Many electrical devices connected to the power grid operate based on the principle of electromagnetic induction. For example, transformers alter voltage and transfer energy through magnetic fields, while motors rotate and drive mechanical loads. The magnetic energy required for these processes is supplied by the power source. Motors and transformers establish alternating magnetic fields during energy conversion; the power absorbed and released within one cycle is equal, and this type of power is called inductive reactive power.
Capacitive Reactive Power: When a capacitor is connected to an AC power grid, the charging power in the first half of a cycle equals the discharging power in the second half. As no energy is consumed, this charging and discharging power of the capacitor is termed capacitive reactive power.
Apparent Power: In an AC circuit, if the load is purely resistive, voltage and current are in phase, and their product equals the active power. However, in circuits containing inductance or capacitance, voltage and current exhibit a phase difference. Therefore, the product of voltage and current does not represent the actual power absorbed by the load but is referred to as apparent power. It is denoted by the letter S. The international unit is the volt-ampere (VA). Typically, apparent power is expressed in kilovolt-amperes (kVA).
Power Factor: The ratio of active power to apparent power. Denoted by cosφ, it is dimensionless. cosφ = P/S.
Power Triangle: The relationship among active power, reactive power, and apparent power conforms to the Pythagorean theorem, as illustrated in the figure.
